Mulch is any material that a gardener lays over the top of the soil. It has numerous advantages compared to leaving the ground bare: it helps retain moisture, suppress weeds, and keep the soil temperature stable (in a cold climate, that can help protect against freeze and thaw cycles, which destroy the soil structure).
Mulches can also improve the soil structure, drainage, and nutrient-holding capacity, and can even help to make a garden look more attractive.
Different Types Of Mulch
There are numerous different kinds of mulch. We can first divide them into organic and inorganic.
Inorganic mulch might include synthetic textiles. These are good at blocking weeds, which is why they’re popular with public gardens and landscaped facilities, but they don’t improve the soil and can get very hot in summer. These textiles are often based on fossil fuels and won’t last more than a few years, so on the whole I’d regard them as a form of pollution rather than a proper gardening technique.
But inorganic mulch might also include rocks or gravel. In some arid areas, rocks are used as mulch around tree trunks; the rocks create a cool, moist microclimate underneath them, retaining moisture for the tree roots. The Pueblo people of the Southwest often mulched with a porous pumice stone in their canyon gardens.
- Organic mulches are more usual. They might actually be living plants, in the form of a cover crop or a living mulch such as creeping thyme or alfalfa sowed between the rows of a vegetable patch. Some of the best-known types of organic mulch are:
- Shredded or chipped bark. This is a good mulch for areas where you won’t do a lot of digging—under trees, in ornamental beds with mainly perennial plants, and also for walkways. The bark provides an attractive surface while keeping moisture in the soil and will eventually break down into organic matter.
- Pine needles. This is a good mulch for deterring weeds. It doesn’t become compacted, unlike some other organic mulches; it keeps moisture in the soil efficiently and breaks down eventually. It can also look very smart.
- Grass clippings have the advantage of being freely available in most gardens, but they can become slimy and smelly if too thick a layer is used and they may mat down and become compacted. They are good to use in areas that aren’t yet planted. Otherwise, leave the clippings on the lawn, or put them in the compost heap.
- Newspaper is easy to use and commonly available. It’s good for smothering grass and weeds if you layer it four to eight sheets thick. When laying it, wet it down so the wind can’t blow the sheets about, then cover it with an inch or two of another organic mulch, or topsoil. It’s easy to cut a hole to plant seedlings and more established plants through the newspaper.
- Straw is a popular mulch for the vegetable garden, and good for putting around fruit and veg that would otherwise trail on the ground, such as squash or strawberries. It will last the whole year round and decomposes slowly. Remember, though, where you’ve used a straw mulch, you may need to add nitrogen next year, or with your cover crop in the winter. Be sure if you buy in the straw that you know where it came from, and that it doesn’t include any chemicals or pesticides. Sourcing from an organic farm is often the best way to ensure you get a completely natural product.
- Shredded leaves or leaf mold may not look right in a formal setting, but if you have an orchard or woodland garden, they can be very suitable. If laid in spring, the mulch will start to blend in nicely and it particularly attracts worms into the soil where it’s laid. It rots down well to provide plenty of organic matter.
- Cardboard, while not the most attractive of mulches, is a superb weed suppressant. If you can lay hands on really big sheets, such as the boxes refrigerators and freezers come in, they can be laid out to recover overgrown areas. Cardboard mulch really earns its keep when you’re starting a garden or a bed. Cardboard mulch ‘collars’ around the bottom of newly planted fruit trees can also keep weeds down and help them get established.
- Coffee grounds. The big disadvantage of coffee grounds is that unless you run a coffee shop, you’re never going to have enough of them to make a big difference. They’re a great mulch for salads and tender plants though, as slugs hate them.
- Wood chips and sawdust can make attractive mulches that will last a while in the garden. The drier and woodier the material, the more slowly it will break down. Wood chip mulch made from waste wood that would otherwise go to the landfill is ecologically a really smart move, and it has very high moisture retention and insulation capabilities. It’s also great for weed control. If you know a tree surgeon, landscaper, or woodworking workshop, you might even have access to some free supplies.
Wood Chip As Mulch
I really like wood chips as mulch. It will protect the soil surface for a long while, improving the infiltration of rainfall, and reducing the evaporation of moisture in the soil. If you get a good, coarse wood chip, it will decay very slowly and that will reduce the impact on the soil’s nitrogen levels (that’s a weakness with wood shavings and sawdust: they break down fairly quickly and since they have a high carbon to nitrogen ratio, the process can result in the soil being depleted of nitrogen).
Wood chip mulch also looks good, since its color is usually similar to that of the soil. That makes wood chips good mulching for ornamental flower beds, compared to (say) cardboard or straw. And laid fairly thickly, it’s one of the best mulches for suppressing weeds because it lets very little light through.
Wood chip feels natural because it is natural. In any woodland or forest, broken twigs and branches form a good part of the forest floor, so this is the way most mature woodlands regulate themselves and provide nutrients and organic matter for the soil. The lignin content in wood chips results in them being mainly broken down by soil fungi; which can provide a substantial enhancement to the soil structure over a couple of years, as the fungi help to bind soil particles together.
Not all wood chip is the same. If you’re looking for a real boost to soil fungi, you’ll want to use wood from deciduous trees, not conifers. And don’t use wood from trees that are toxic to other plants, like walnut and red cedar. (On the other hand, pine cedar and cypress chips apparently repel many pests, such as fleas, ticks, and gnats.) Make sure the wood hasn’t been chemically treated.
If you have a chipper, it’s quite easy to make your own woodchip from small branches and twigs. In fact, smaller branches are more nutritious than tree trunks, so this is some of the best compost you can get; if you have a little fruit orchard, your prunings can be used as mulch. Remember, though, to stop the mulch an inch or so away from the trunks of trees (and the stems of other plants, too).
A living mulch is a low-growing or creeping plant that’s used to cover the ground between crop plants. For instance, creeping thyme makes a good ground cover as well as a good companion plant for many vegetables. Red clover, alfalfa, calendula, and sweet alyssum all make good living mulches.
When you use living mulch, you need to think about what kind of plants it’s going with and what other qualities it has. For instance, if you want to deter pests from a tomato crop, calendula would make a good mulch. Clover is good for attracting pollinators, while alfalfa is a great nitrogen fixer.
Many herbs make good living mulches and can also be picked for your kitchen. Dill, thyme, and camomile all do well. But don’t use mint: it’s invasive.
While most gardeners wouldn’t call it a living mulch, a good, strong cucumber or gourd plant will act in the same way as a mulch. Its huge, sprawling leaves will suppress any weeds that try to come through and keep the moisture in the soil. Once you’ve finished harvesting, you can either chop the vine up and leave it where it is or cart it off to the compost heap.
Green manure involves growing a plant that you will chop down and use as a soil fertilizer. It’s most efficient in terms of the amount of work you have to put in if you grow green manure where you want to use it; then you just have to chop it down, chop it into the soil a bit, and leave it where it is to decay. That’s very similar to using a cover crop. But you can grow green manure elsewhere if you want to; you’ll just need to use your wheelbarrow and a bit of effort.
You’ll usually want a plant that will grow quite quickly over the fall and winter period and which will be ready to chop and use next spring. Winter grazing rye is a good choice for this. But quick-growing mustard can be sown in September and incorporated into the soil just a month or two later (or you can just leave it chopped down as mulch). A good summer choice is a fenugreek, which has abundant leaves to suppress weeds and is also a herb for Indian cooking (use either fresh or dried leaves, or seeds).
Leave the site for at least two weeks, or more if you can, after you’ve chopped green manure into the soil before you sow or plant a new crop. Other plants you might consider for green manures are:
- Blue lupin (Lupinus angustifolius), sown in spring or early summer for digging in two or three months later
- Summer-sown buckwheat
- Red clover, sown up to the end of August—overwinters well
- Crimson clover, sown in spring or summer—needs digging in after a couple of months
- Winter field beans (Vicia fava) sown in fall—can be dug in when they flower
- Phacelia (summer and fall)
Non-living mulches include all kinds of plastics and synthetic mulches. Most of these are manufactured from fossil fuels, so on principle, if you care about the environment, you really don’t want to use the ones comprised of man-made materials. They will also decay after some years, leaving tiny shreds of plastic in the soil and allowing stronger perennial weeds like bamboo or couch grass to start penetrating the cover layer.
However, rock or stone can make good non-living mulch, particularly if you live in a rocky area. This is a traditional way to help retain moisture around fruit trees in hot climates, piling rocks on the ground within the tree’s drip zone. You could also use slates or large pebbles as mulch for ornamental beds.
Gravel is also a possible mulch, but it has disadvantages. It will gradually disappear into the soil, making it stony, which is not particularly desirable. Most store-bought gravels come from industrial quarries, often leading to the destruction of landscape, and there may be a considerable carbon cost involved in their processing and transport.
Sheet mulching is a great way to protect your soil by completely covering it with mulch. However, you have to get your timing right. In spring, you want the soil —exceptionally—to be bare so that it can warm up quickly, and so that perennials that need to push new shoots up through the soil have an easy time. You might even take away what’s left of last year’s mulch.
But once the garden has got going again, you will want to mulch properly, to stop weeds coming through and to keep the moisture in the soil for your growing plants.
Then in the fall, you’ll want to mulch (unless you plan to plant a cover crop) to protect the soil from erosion and compaction over the winter. It’s best not to add new mulch on top of the old but instead to scrape the old mulch away and then put down a new layer. Old mulch might harbor fungal disease, for instance; by taking up the old mulch, you stop that from being passed on to the new mulch. Of course, the old mulch can now go in the compost, preferably in a good hot composting heap.
Sheet mulching is a particularly good way of starting a new garden or vegetable bed without having to dig. Simply cover the area with a deep sheet of mulch over the fall and winter, and you’ll be able to eradicate many perennial weeds and attract earthworms and fungi which can start improving the soil. See the box for further detail on how to sheet mulch.
How to sheet mulch
The day before you lay the mulch, give the whole area a real soak with water. The next day, cover the entire ground with a layer of cardboard, or you could use old newspapers. Make sure you overlap the edges, then soak the cardboard or paper.
On top of that layer, put organic matter such as food scraps, grass clippings, manure, straw, or shredded leaves. You want this to provide nitrogen, so remember this needs to be a ‘green’ layer. You could even use chopped comfrey or the green parts of prunings from the garden if you’ve been chopping a lot of stuff down. Add any soil amendments you want, like bloodmeal, kelp, or lime.
Then cover that with a second layer of ‘brown.’ This could be straw, dead leaves, bark chippings, wood shavings, or more cardboard. Again, give it a good soak: water it until it’s really saturated and heavy.
On top of this, scatter a nice layer of a couple of inches of compost. This is what you’ll plant seeds into directly, so the quality is important. Use good compost; it’s tempting to use topsoil from elsewhere on your plot, but you run the risk of introducing weeds.
Cover over the compost with another carbon layer such as bark chips; this helps lock in all that moisture and it also stops weeds from getting started in the compost. Another soaking will help soften the cardboard, and it will also attract earthworms. If you want to, you could even now sow green manure like clover, which will help break down the materials in the bed and will fix nitrogen. Note that not all green manures will fix nitrogen, only leguminous ones, so clover should preferably be chosen.
Other leguminous green manures include cowpeas, chickpeas, lentils, fava beans, alfalfa, fenugreek, green peas, and soybeans.
Sheet mulching properly needs a lot of organic matter. A fifty-square-foot patch will need an entire pickup truck full of straw (or other mulch). You might want to start small, either centering your mulch on a fruit tree or planting really close to the house to make your first vegetable bed. There are two crucial parts to success. First, make the layers thick. Second, get them really, really soaked with water. Other than that, sheet mulching is quite forgiving. If you don’t have exactly the right ingredients for each layer, improvise.
If you can leave it for three months, you’ll have marvelous soil, so it’s a great thing to do in the fall and leave it with an over-wintering crop of green manure. Then it will be ready for spring planting. Or you can plant seedlings by just digging a hole through the cardboard.
How does it work? The cardboard does two things. First of all, it deprives the weed seeds of light, so if perennial weeds try to grow, they won’t be able to force their way through the cardboard. Secondly, as it rots down, it will attract earthworms and other soil organisms to break down the organic materials in the mulch.
By the way, if you’re doing permaculture on a shoestring, look for free supplies; try looking for supermarkets or other outlets that have cardboard boxes going begging or a farm that has a waste pile of used straw.
A hügelkultur or ‘mound culture’ is a way of using big pieces of tree you’ve chopped down to create a vegetable bed. Dig a big trench and fill it with tree trunks, branches, and twigs. You can overfill it a bit (in fact, some people just mound up the wood and don’t dig a trench, but if you do this, remember to lay a cardboard sheet of mulch to suppress perennial weeds before you add the wood).
Then take the turf you cut away and put its root side up over the woody material. Over that, put down a layer of leaves and soil, then a layer of half-rotted compost, then a layer of fully-rotted compost, and then straw. Plant seedlings such as squash and tomatoes through the straw.
As the wood rots, the mound will gradually become lower. The wood retains a huge amount of moisture, which makes hügelkultur a great technique for growing moisture-loving veg in drier climates.
In fact, the hügelkultur mimics the forest floor, where you’ll often see vegetation like brambles and fungi growing on rotting, fallen wood. The wood will break down over several years, feeding the nutrients continually into the soil—it’s a slow-release method. When wood is kept wet, it becomes soft and breaks down more easily into organic matter. It will also house mycorrhizal fungi, which will help stimulate further microbial activity and assist the process of soil aggregation.
One warning: don’t ever try to make a hügelkultur next to a swale. The wood is buoyant and that will be a weak point in the berm, which could breach. If you ake hügelkulturs on a slope, run them up and down the slope, not across it.